Apart from the unfortunate fact that farm gates tend to get left open by unknowledgeable or careless people, it is a tiresome thing to have to open and close gates to enable passage of a horse or a motor vehicle.
"Automatic" gates are known and generally fall into two categories--boom gates and gates which move into a horizontal position on the ground. Both kinds are mechanically complex and expensive--one boom-operated 12 foot gate being presently on the market for over $A1000. Moreover a person on foot or horseback will have difficulty in operating the trip mechanism and often a separate wicket is provided beside the main gate.